In copending U.S. patent application Ser. No. 489,757, filed Apr. 29, 1983 and entitled "Mobile Offshore Drilling Structure for the Arctic", there is disclosed a mobile platform structure having particular application to exploratory drilling for oil and gas in the substratum underlying offshore acrtic waters and ice formations. The mobile platform structure includes an ice load bearing, submergible substructure and a platform superstructure of approximately equal lateral dimensions, the latter being supported on the substructure and above sea level when the substructure is ballasted onto the sea floor. The substructure has a height approximately equal the desired depth of submergence and includes horizontal top and bottom walls, a substantially vertical, peripheral side wall surrounding the top and bottom walls, and a plurality of vertical bulkheads extending between the top and bottom walls to form a plurality of ballast compartments.
The mobile platform structure of such copending application incorporates large diameter spuds to provide a large proportion of the structure's lateral resistance to global lateral ice loads. The spuds are relatively long and extend vertically through vertical spud guides including spud sleeves interposed between the vertical bulkheads and connected top and bottom to the top and bottom walls of the substructure. The spuds consist of large diameter steel cylinders which are reactively supported by bushings at the top and base of the substructure for lateral load transfer to the top and bottom walls of the substructure.
During the application of global lateral ice loads, the structure will displace until equilibrium is achieved between such loads and the resisting forces from the mobilization of the foundation soils by the mat (base of the substructure) and the spuds. The spuds may elastically yield in bending until such equilibrium is established. Also, the spud bushings will permit relative vertical movement between the spuds and the structure so that the structure can maintain contact with the soil during any consolidation settlements and tilt slightly as necessary to develop bearing resistance to overturning moments without overloading the structure locally at spud support points.
Upon redeployment of the mobile platform structure to another installation site, there could remain a substantial residual load in a spud even though the structure then is free of external ice loads. Although the bushings disclosed in the aforesaid application could be removed from their mounts, as by jacking, to relieve a residual load, such would have been difficult and time consuming to avoid possibe damage to the structure or harm to personnel. Analyses of the spuds have indicated that the bushing loads could be very high for a fully loaded spud. Displaced spud profiles indicate that, for a fully elastic spud, the spud head may "kick back" laterally if the upper bushing, that at the top wall of the substructure, is removed under full or substantial residual load.
Accordingly, it is important to provide a spud bushing system which avoids damage to the structure and personnel while permitting quick and efficient removal of the bushing for alleviating high residual loads in the spuds.